Merge branch 'nav-scroll-bug' into 'master'

Side nav scrolls if content height taller than screen
## What does this MR do?
Header logo is now fixed at top; put back scrolling functionality in side nav
## Screenshots (if relevant)
<img src="/uploads/e1d1b60c296c39c61d7fe7da22b6c52d/scroll.gif" width="600px">
See merge request !4513